1. Field of the Invention
The present invention relates to an information recording disc and an information recording/reproducing apparatus.
2. Description of the Prior Art
Conventionally, an information recording disc and an apparatus for recording and reproducing an information signal using the information recording disc have been suggested. As such an information recording disc, there can be exemplified a so-called optical disc, a magneto-optical disc, a hard disc, and the like.
The so-called magneto-optical disc includes, as shown in FIG. 1, a signal recording layer (MO film) 101 which is sandwiched by dielectric films 102 and 103 having an enhancement effect. The signal recording layer 101 and the dielectric films 102, 103 are protected by a protection film 104 and the substrate 105 and a cover material, respectively, and they are not exposed outside. Accordingly, in this magneto-optical disc, there is no danger of contact or collision of the signal recording layer 101 and the dielectric films 102, 103 with an objective lens or a magnetic head of an optical pick-up apparatus, and there is no need of countermeasures for preventing such contacts or collisions.
Moreover, in the so-called hard disc, when reproducing an information signal recorded or recording an information signal onto the information recording disc, as shown in FIG. 2, it is preferable that the signal recording layer 101 formed on the substrate 105 of the information recording disc be in the vicinity of the magnetic head 106, requiring no optical condition. Accordingly, a lubrication film 107 coated with a thin film of a lubricant is formed between the magnetic head 106 and the information recording disc. The lubricant often used contains fluoride for its characteristics. The magnetic head 106 is mounted on and supported by a slider 108.
Recently, in the case of optical disc also, as in the hard disc, an optical disc apparatus in which the signal recording layer is in the proximity with the objective lens of the optical pick-up apparatus has been developed. As such an optical disc, a so-called “optical hard disc apparatus” and a “near-field optical disc apparatus” have been suggested. In the optical hard disc apparatus, an object lens is mounted and supported by a slider. In the near-field optical disc apparatus, an objective lens have a numerical aperture (NA) not smaller than 1 although not having the configuration of mounting the objective lens by the slider.
As shown in FIG. 3, in the “near-field optical disc apparatus”, the objective lens is a so-called solid immersion lens (SIL) 110 whose first surface is arranged in the proximity of the surface portion of the information recording disc and the first surface of the SIL 110 is arranged in a so-called near-field region with respect to that surface portion.
An information recording disc used in such an “optical hard disc apparatus” includes, as shown in FIG. 4, a reflection film 109, a second dielectric film 103, a signal recording layer 101, and a first dielectric film 102 successively arranged from the side of the substrate 105. The information recording disc having the four-layered films consisting of the reflection film 109, the second dielectric film 103, the signal recording layer 101, and the first dielectric film 102 is designed so as to obtain the enhancement effect for increasing a recording data contrast for a light incident vertically to the substrate 105.
In the information recording/reproducing apparatus configured as the so-called “optical hard disc apparatus”, the signal recording layer of the information recording disc is in the proximity of the objective lens of the optical pick-up apparatus and there is a possibility that the signal recording layer collides into the objective lens. Here, when the signal recording layer is coated with a lubricant, if there is any film thickness irregularity in the coating, this causes irregularities in the optical characteristic. Accordingly, it is necessary to reduce the film thickness irregularities.
Moreover, as the lubricant for the near-field optical disc apparatus, a fluoride-system material is not appropriate because it has a low refractive index. Thus, in the near-field optical disc apparatus, it is difficult to select an appropriate lubricant.
Moreover, in the optical hard disc apparatus, it is preferable that the first surface of the objective lens including a boundary in the proximity be coated with an anti-reflection coating (AR coating), so that fluctuations of a distance between the information recording disc and the objective lens will not affect the optical characteristics.
However, since the objective lens is in the proximity of the information recording disc, the objective lens easily collide into the information recording disc and such collision may peel off the anti-reflection coating. If the anti-reflection coating is peeled off from the objective lens, the optical characteristic of the entire apparatus is changed, disabling a preferable recording/reproducing of an information signal. Thus, it is difficult to select a film formation material of the anti-reflection coating as well as to handle the coating.
Furthermore, in the aforementioned configuration, among a light flux applied to the information recording disc, a component having a high numerical aperture (NA) cannot reach the signal recording layer easily. That is, assume that an air film between the information recording disc and the SIL is a thin film having a refraction index (n) of 1, the calculation result of dependency of the reflectance of the first dielectric film on the incident angle when incident via an air gap having a thickness t from the SIL as an optical part having a refractive index (n) of 1.8 shows that different incident angles corresponding to the numerical apertures fluctuate the intensity of the light incident into the first dielectric film as shown in FIG. 5. Furthermore, when the optical system is designed for an air gap having a thickness of 50 nm, the fluctuation of the air gap thickness drastically changes the reflectance, easily causing fluctuation of the light energy distribution.
Moreover, as shown in FIG. 6 and FIG. 7, a calculation result of the modulation transfer function (MTF) shows that increase of the air gap thickness between the information recording disc and the SIL easily deteriorates the MTF.
Furthermore, in the information recording disc having the film configuration shown in FIG. 4, the outermost surface opposing to the SIL is a film made from ZnS—SiO2. This ZnS—SiO2 is a very soft material and is easily damaged when a collision with the SIL occurs. In the hard disc, polishing is performed to improve the surface characteristic. However, when the outermost surface is made from a soft material such as ZnS—SiO2, polishing cannot be performed.
Moreover, in this information recording disc, an abrasion phenomenon is easily caused during an information recording, which causes a problem that the film is removed and rubbish is easily generated.